Iron and quorum sensing coordinately regulate the expression of vulnibactin biosynthesis in Vibrio vulnificus

Vibrio vulnificus is a halophilic marine pathogen associated with human diseases such as septicemia and serious wound infections. Genes vvsA and vvsB, which are co-transcribed and encode a member of the nonribosomal peptide synthase family, are required for vulnibactin biosynthesis in V. vulnificus. In this study, we found that quorum sensing represses the transcription of a vvsAB-lux reporter fusion. Gel shift assay and DNaseI footprinting experiments show that the main regulator of quorum sensing, SmcR, binds to a 22-bp region located between -40 and -19 with respect to the vvsA transcription start site. Mutation of the SmcR binding site abolishes the repression of vvsA::luxAB by SmcR. Fur represses vvsAB transcription in the presence of iron by binding to a 47-bp region located between -45 and +2 with respect to the vvsA transcription start site. A competition gel shift assay and footprinting experiment using Fur and SmcR showed that Fur binds to the vvsA promoter region with higher affinity than SmcR. Studies with the vvsAB::luxAB transcriptional fusion demonstrate that in the presence of iron, Fur is the key repressor of vvsAB transcription, whereas in iron-limited conditions, SmcR is the key regulator repressing vvsAB transcription. This study demonstrates that the Fe-Fur complex and quorum sensing cooperate to repress the transcription of vvsAB in response to iron conditions, suggesting that fine tuning of the intracellular iron level is important for the survival and pathogenicity of V. vulnificus.